1. Field of the Invention
The present invention is related generally to medical electrical leads. More specifically, the present invention is related to implantable electrical leads. The present invention may find use in implantable neurological devices, cardiac devices, as well as many other devices.
2. Prior Art
Implantable leads having externally exposed ring or band electrodes can be used to deliver electrical stimulation to surrounding tissue and/or to sense electrical energy produced by the surrounding tissue. Such leads are often implanted, for example, within the epidural or intrathecal spaces of the spinal column, along peripheral nerves, within the brain, and about the heart. Electrical stimulation of the spinal cord has been shown to be effective in relieving intractable pain in some patients. Such electrical stimulation can reduce or eliminate the use of pain relieving drugs. Examples of some leads may be found in U.S. Pat. Nos. 6,721,604; 6,981,314; 6,216,045; and 5,483,022, herein incorporated by reference.
One such lead is formed of polymeric material, for example, polyurethane or silicone. The lead can be nominally 1 mm in outer diameter and about 20 cm in length. A typical lead may have a series of electrodes formed as bands or rings disposed in a spaced apart relationship in a lead distal region. The distal region of the lead can be introduced, for example, into the epidural region for use in stimulation of the spinal column. The lead proximal region may have a corresponding set of band or ring connectors or terminals, one for each corresponding electrode in the distal region. Each proximal region terminal can thus be connected to one distal electrode in a typical configuration.
The terminals can be used to couple the proximal end of the lead to a lead extension, which can in turn be coupled to an implantable pulse generator (IPG). The lead extension can provide added length to extend the reach of the lead to a more distantly placed IPG. In some embodiments, the lead extension is between about 20 and 50 cm in length.
The lead typically has a lumen extending from the proximal end through to the distal region, with the lumen being dimensioned to accept a stiffening member or stylet. The lead, commonly formed of a polymeric material and being very small in cross section, is typically very floppy and not pushable. With a stylet or stiffening member inserted, the lead gains the needed pushability, and can be advanced into and up the spinal column to the desired location.
Small size, in particular, small outer diameter, is desirable for nerve stimulation leads. A small profile is less intrusive and may be easier to deliver. Some peripheral nerve stimulation therapies would benefit from small profile leads. Flexibility is desirable for both ease in delivery and for a more comfortable patient experience, as the patient may be aware of a stiffer implanted lead. A long flex life, the ability to survive a large number of lead flexures over the expected lead lifetime, is also desirable. Some cardiac leads are subjected to flexure with every heart beat. Peripheral nerve stimulation leads may be flexed and twisted with the patient's movements.
Some leads have multiple, electrically independent electrical conductors. Spinal cord stimulation leads include several, often eight, conductors. The conductors are often not arranged haphazardly. A straight or coiled cable of conductors is often used. Issues arise that relate to the orientation and spacing of the conductors as they tend to float inside the lead body tubing. As the conductors float and potentially cross each other, flex life can be negatively impacted.
Different configurations have different benefits and different draw backs. Using straight conductors is not optimal, as they break. The smaller the size, generally, the better the conductor flex life. Also, it is hard to make a coil out of a cable, as there is need to put a great deal of stress on the cable as it is wound around a mandrel in order to make the coil stay after the mandrel is removed. One common conductor uses a 7×7 conductor having seven wires, each having seven filars in a twisted rope-like configuration.
What would be desirable is an arrangement for multiple conductors in a lead that provides for a long flex life and minimizes the float and potential crossing of multiple conductors.